How do you interpret an Arterial Blood Gas (ABG) result?

ABG Interpretation: A Systematic Approach

Interpret arterial blood gas results using a three-step systematic method: first assess pH to determine acidemia versus alkalemia, then evaluate PaCO2 for the respiratory component, and finally examine bicarbonate/base excess for the metabolic component. 1

Step-by-Step Interpretation Algorithm

Step 1: Assess the pH

• pH < 7.35 indicates acidemia 1
• pH > 7.45 indicates alkalemia 1
• This first step determines the primary direction of the acid-base disturbance 2

Step 2: Evaluate the Respiratory Component (PaCO2)

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
• PaCO2 directly reflects ventilation status and can identify acute versus chronic respiratory failure 3

Step 3: Evaluate the Metabolic Component

• Base excess < -2 or HCO3 < 22 indicates metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 indicates metabolic alkalosis 1
• The metabolic component reflects kidney function and metabolic processes 2

Clinical Context for ABG Testing

When to Order an ABG

Critical indications requiring immediate ABG:

• All critically ill patients to assess oxygenation, ventilation, and acid-base status 1, 4
• Shock or hypotension 4, 5
• Oxygen saturation fall below 94% on room air or supplemental oxygen 4, 5
• Suspected diabetic ketoacidosis, metabolic acidosis from renal failure, trauma, or sepsis 1, 5
• After return of spontaneous circulation following cardiopulmonary resuscitation 5
• Carbon monoxide poisoning (pulse oximetry will be falsely normal) 5

Important caveat: A normal oxygen saturation does not rule out the need for ABG measurement, especially in patients on supplemental oxygen or those with potential acid-base disturbances 4, 5

Management Based on ABG Results

Respiratory Acidosis Management

• Initiate non-invasive ventilation (NIV) for acute hypercapnic respiratory failure with pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) despite optimal medical therapy 1
• Start controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1
• Repeat ABG after each titration to monitor for worsening hypercapnia 1, 4

Oxygen Therapy Titration in At-Risk Patients

• For patients with COPD or known CO2 retention, start with low flow oxygen (1 L/min) and titrate up in 1 L/min increments until SpO2 >90% 4
• Obtain ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration 4, 5
• After each oxygen flow rate adjustment in patients with baseline hypercapnia, perform ABG analysis 4, 5

Special Population Considerations

COPD Patients

• Check ABG when starting oxygen therapy, especially with known CO2 retention 1, 4
• Patients who develop respiratory acidosis (rise in PaCO2 >1 kPa or 7.5 mm Hg) during oxygen therapy have clinically unstable disease and require further medical optimization 4

Hepatopulmonary Syndrome Diagnosis

• Use P(A-a)O2 ≥ 20 mmHg cutoff instead of ≥ 15 mmHg for patients aged ≥ 65 years 1
• PaO2 < 80 mmHg or P(A-a)O2 ≥ 15 mmHg (≥ 20 mmHg if age ≥ 65) establishes the diagnosis 1

Common Pitfalls to Avoid

• Do not rely solely on pulse oximetry: Normal oxygen saturation does not exclude significant acid-base disturbances, hypercapnia, or anemia 4, 5
• Do not overlook metabolic conditions: Patients with breathlessness may have diabetic ketoacidosis or metabolic acidosis requiring ABG analysis 4, 5
• Do not skip repeat measurements: Always repeat ABG after changes in oxygen therapy, especially in patients at risk for CO2 retention 4, 5
• In carbon monoxide poisoning, pulse oximetry readings are falsely normal, making ABG essential 5

Technical Considerations

• Use local anesthesia for all ABG specimens except in emergencies 4, 5
• Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand 4, 5
• For most non-critical patients, arterialized earlobe blood gases may be used to measure acid-base status and ventilation, though PO2 is less accurate 4, 5